[SDA] Don't stop divergence propagation at the IPD.

Summary:
This fixes B42473 and B42706.

This patch makes the SDA propagate branch divergence until the end of the RPO traversal. Before, the SyncDependenceAnalysis propagated divergence only until the IPD in rpo order. RPO is incompatible with post dominance in the presence of loops. This made the SDA crash because blocks were missed in the propagation.

Reviewers: foad, nhaehnle

Reviewed By: foad

Subscribers: jvesely, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65274

llvm-svn: 372223